Brief report: Asperger's syndrome and sibling birth order.

Schmidt et al. (2013) looked back at the charts of 29 people who had been sent to a psychiatry clinic and were diagnosed with Asperger syndrome.

They simply counted how many of these patients were the first-born child in their family.

Eighty-six percent of the referred Asperger patients were first-borns. That is far higher than the fifty percent you would expect by chance.

The authors say this shows a clear over-representation of first-borns in the referred group.

Root et al. (2017) seems to disagree. They found that preschoolers picked up by infant-sibling research were more likely to be girls, not first-borns. The key difference is how the kids were found. Karmen’s study used clinic referrals, while R et al. used baby-sibling screening. Referral bias can make first-borns look more common, because parents notice quirks sooner when there is no older child to compare with.

Levin et al. (2014) extend the story. They showed that when the child with ASD is older, their behavior problems spill over to the neurotypical sibling. Birth order here acts as a moderator, not just a head-count.

Tanidir et al. (2014) give a conceptual replication. They also tracked a referred Asperger sample, but focused on the two-year delay between first referral and final diagnosis, not on birth order. Together these papers warn that clinic samples can look different from community samples.

If you work with clinic-referred clients, remember that first-borns may be over-represented. Do not assume the same pattern holds in community-screened kids. When you read studies, check how the sample was found—referral, research, or screening—before applying the numbers to your caseload. On Monday, add a quick question about birth order and referral source to your intake form. It helps you spot referral bias and plan sibling support when the ASD child is older.